The present invention relates in general to electric motors having an electronic commutator and a frequency generator or tachogenerator providing a signal used in the speed control of the motor and more particularly to tachogenerator construction for such electric motors to provide the signals used for speed control.
The invention in general relates to direct drive brushless motors and associated control systems for providing high accuracy speed control of the motors, for use for example on floppy discs and similar applications. In general, the brushless motor includes a permanent field magnet secured to a rotor, the magnet being magnetized with regularly arranged magnetic poles of alternating polarity, together with a stator having a plurality of driving coils interlinked with the magnetic charges of the magnet. A speed detecting coil is disposed in the field of the magnet having a voltage induced by the magnetic poles. The typical prior art approach has been to create a tachometer output signal by such an arrangement which presents a high frequency. This frequency is proportional to the revolution speed of the rotor. Any change in the moving speed of the rotor would cause a frequency modulation that can be transformed into a voltage modulation through an analog speed control circuit which is able to adjust the input power to the motor in order to keep the speed constant. The analog circuit converts the frequency into a voltage affected by a certain amount of ripple which needs to be averaged in order to provide a DC voltage output. The frequency of the tachometer must be at least 10-20 times the highest frequency response desired in order to provide enough accuracy to the speed control of the motor. However, the high frequency system has certain shortcomings. Notable among these are that the high frequencies are generated by additional provision of a magnet on the rotor outside diameter which substantially increases the size of the motor. Also it is costly to produce a high frequency tachometer with no real benefit because the averaging operation slows down the response of the speed control.
The electric motor herein described is designed to provide an economic method of controlling the commutation and the speed of the motor by using a minimum amount of motor built-in sensors, which are connected to a digital signal processing circuit. The digital processing circuit is designed to maintain the speed of the motor to a predetermined value with a high accuracy such as is required in data processing equipment for floppy discs and the like. The electric motor with the novel tachogenerator structure of the present invention is designed to provide a low frequency signal to a digital control circuit which can maintain a high degree of accuracy in controlling speed because it uses a low frequency tachometer that has a low jitter content and updates the speed after each tachometer input without averaging. Thus the digital circuit measures the time between the tachometer inputs very accurately and performs better with a lower frequency and lower jitter signal, so that the calculation time is negligible when compared to the sampling rate and the jitter does not induce errors into the feed-back loop. Such low frequency tachometer signals are inexpensive and simple to generate in the motor.
In typical tachogenerators used in prior art, one conventional type of structure uses a tachometer that extends around the outer periphery of the motor. In this arrangement, the motor magnet is an annular structure arranged above the motor coil concentric with a common vertical axis, with an inverted cup shaped structure fixed to the motor magnet, supporting an annular tachometer magnet which encircles the periphery of the motor and is disposed over the conductor pattern of the tachometer coil. Other prior art, for example FIG. 1 of U.S. Pat. No. 4,385,249, shows a tachogenerator that extends on the side of the motor with the tachometer coil extending in a concentric path around the tachometer magnet mounted on the rotor shaft and located substantially midway between the axis of the shaft and the perimeter of the motor. Still another version is shown in FIG. 2 of U.S. Pat. No. 4,385,249, which employs the tachogenerator formed of the coil and rotatable tachometer magnet located inside the center or opening of the annular motor magnet. Still another version shown in U.S. Pat. No. 4,109,170 employs a tachogenerator that is inserted between the magnet and the coils and therefore decreases motor torque. A similar version of this structure is shown on FIG. 1 of U.S. Pat. No. 4,260,920 where the tachometer poles are magnetized on the outer periphery of the motor, thus further reducing the motor torque as will be explained in the detailed description.
The above arrangements have certain drawbacks which have now been recognized. They involve added costs due to the use of two separate magnets which must be assembled on the rotor, and usually increase overall motor dimensions. They provide poor motor performance in the case of a tachogenerator involving only the main magnet since necessitating a large air gap between the magnet and the motor coils to insert the tachogenerator coil.
An object of the present invention is the provision of a low speed direct drive brushless motor, for floppy discs and similar data processing applications, having a novel tachogenerator structure wherein the magnet of the motor is magnetized with two tracks of multipolar magnetization, providing a main portion disposed in an annular path around the outer portion of the magnet magnetized to provide the motor poles, and a small track extending in an annular path along the inner circumference, avoiding the drawbacks of the above described prior art structures. The inner magnetized track providing the poles for the tachogenerator operation does not bring effective magnetic charges which would interfer with the flux created by the coils to generate torque and is magnetized with a predetermined number of poles that is higher than the number of motor poles, creating the rotor portion of the tachogenerator. The stator portion of the tachogenerator is provided with an equal number of poles as the magnetic poles on the inner track of the magnet. The equal number of poles on the rotor magnet and the stator creates an averaged induced alternating signal which has very low jitter or frequency variation. Thereby, the desired low frequency tachogenerator signal is produced having a low jitter content, which can be processed by a digital control circuit to measure the time between the tachometer inputs very accurately and wherein the calculation time is negligible when compared to the sampling rate, such that jitter does not induce errors into the signal processing.
Other objects, advantages, and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.